The invention relates to gas turbine engine aerofoils and particularly to aerofoils for stators and rotors in the low pressure turbine of an aero engine.
Low pressure turbine aerofoils generally do not require any provision to be made in the aerofoil shape for the inclusion of a cooling system. This is because the low pressure aerofoils operate in an environment which is relatively cool compared to that of the intermediate and high pressure aerofoils.
One known design of low pressure aerofoil is referred to as xe2x80x9cthick hollowxe2x80x9d. The aerofoil is manufactured using a lost wax process in which wax aerofoils are produced in a master dye with a ceramic core within the wax. A shell is formed about the wax and molten metal injected into the shell at locations along the span of the aerofoil. After casting, the ceramic core within the metal aerofoil is leached out leaving much of the aerofoil with a very thin wall section and therefore achieving minimum weight. In some cases the wall section is the minimum required for mechanical (stress) reasons.
The low weight of thick hollow aerofoils is highly advantageous. However, there are two drawbacks with the above manufacturing process. Firstly, the use of a ceramic core significantly adds to the cost of manufacture. Secondly, injection into the mould results in xe2x80x9cgatesxe2x80x9d of excess metal standing out from the aerofoil surface, which have to be dressed by an additional machining operation. The steps or discontinuities in the surface that are left may result in loss of aerodynamic performance. In order that the gates may be accessed for removal, they are required to lie on the convex suction surface of the aerofoil, whose shape is much more critical to its aerodynamic performance than is the shape of the pressure side.
An alternative design of low pressure aerofoil is referred to as xe2x80x9cthin solidxe2x80x9d. The manufacturing process is generally similar to that for thick hollow aerofoils. However, there is no ceramic core and the metal is injected into the mould from the ends of the aerofoil. This prevents the formation of unwanted gates on the aerofoil surface. However, there is a drawback in that the aerofoil must be of sufficient thickness to allow the metal to flow fully to the mid-span section of the aerofoil. This thickness is greater than that needed for mechanical reasons (i.e. to have acceptably low stresses) and as a result the aerofoil is heavier than necessary, and certainly heavier than a thick hollow version of the aerofoil.
According to the invention there is provided an aerofoil for a gas turbine engine, the aerofoil being substantially solid and including a concave pressure surface and a convex suction surface, wherein the pressure surface is provided with a projection extending therefrom.
Preferably the aerofoil is elongate and is adapted to be oriented in a generally radial direction of the gas turbine engine and the projection comprises an elongate fin extending in the radial direction of the aerofoil. The elongate fin preferably extends substantially along a whole radial span of the aerofoil.
The aerofoil may have a varying cross-sectional thickness, being thicker in a central region thereof and tapering towards its edges and the projection may extend from the pressure surface at a central, relatively thick region of the aerofoil. The projection may extend from the pressure surface in a direction substantially perpendicular to a tangent to that surface.
The aerofoil may include radially extending leading and trailing edges joined by the pressure and suction surfaces and a disturbed flow region defined between the aerofoil pressure surface and a plane extending tangentially to a pressure side of the leading edge. The projection is preferably located fully within the disturbed flow region. The projection may extend from the pressure surface across between 25% and 100% of the disturbed flow region.
Preferably a cross-sectional thickness of the projection is at least equal to a minimum cross-sectional thickness of the aerofoil.
The cross-sectional thickness of the projection may be substantially uniform. Alternatively, the cross-sectional thickness of the projection may decrease from a proximal to a distal part thereof.
The aerofoil may form part of a low pressure turbine or stator blade for a gas turbine engine.
According to the invention there is further provided a gas turbine engine including an aerofoil according to any of the preceding definitions.
According to the invention there is further provided a method of casting an aerofoil according to any of the preceding definitions, the method including the step of injecting metal into a casting shell via the projection.